Non-destructive testing is the name commonly used to identify any of a wide variety of techniques that may be utilized to examine materials for defects without requiring that the materials first be destroyed. Such non-destructive testing of materials is advantageous in that all materials or products may be tested for defects. That is, after testing, acceptable (e.g., substantially defect-free) materials may be placed in service, while the defective materials may be re-worked or scrapped, as may be required. Non-destructive testing techniques are also advantageous in that materials already in service may be tested or examined in-situ, thereby allowing for the early identification of materials or components that may be subject to in-service failure. The ability to test or examine new or in-service materials has made non-destructive testing techniques of extreme importance in safety or failure sensitive technologies, such as, for example, in aviation and space technologies, as well as in nuclear power generation systems.
One type of non-destructive testing technique, generally referred to as positron annihilation, is particularly promising in that it is theoretically capable of detecting fatigue damage in metals at its earliest stages. While many different positron annihilation techniques exist, as will be described below, all involve the detection of positron annihilation events in order to ascertain certain information about the material or object being tested.
In one type of positron annihilation technique, positrons from a radioactive source (e.g., 22Na, 68Ge, or 58Co) are directed towards the material to be tested. Upon reaching the material, the positrons are rapidly “thermalized.” That is, the positrons rapidly lose most of their kinetic energy by collisions with ions and free electrons present at or near the surface of the material. After being thermalized, the positrons then annihilate with electrons in the material. During the diffusion process, the positrons are repelled by positively charged nuclei, and thus tend to migrate toward defects such as dislocations in the lattice sites where the distance to positively charged nuclei is greater. In principle, positrons may be trapped at any type of lattice defect having an attractive electronic potential. Most such lattice defects are so-called “open volume” defects and include, without limitation, vacancies, vacancy clusters, vacancy-impurity complexes, dislocations, grain boundaries, voids, and interfaces.
Complete annihilation of a position and an electron occurs when both particles collide and their combined mass is converted into energy in the form of two (and occasionally three) photons (e.g., gamma rays). If the positron and the electron are both at rest at the time of annihilation, the two gamma rays are emitted in exactly opposite directions (e.g., 180° apart) in order to satisfy the requirements of the conservation of momentum. Each annihilation gamma ray has an energy of about 511 keV, the rest energies of an electron and a positron. In positron annihilation techniques nearly all the positrons are at rest in the defect or lattice sites. However, the electrons are not. Therefore, the momentum of the electron tends to determine the momentum of the annihilating pairs and cause the direction of the gamma rays to deviate from 180°. In addition to the momentum constraints, the energies of the gamma rays resulting from the annihilation may deviate slightly from 511 keV, depending on the momentum of the electron. Accordingly, in non-destructive testing techniques utilizing positron annihilation, the detection of the energies and relative angles of the gamma rays produced by the annihilation event are used to derive certain information relating to defects and other characteristics of the material or object being tested.
While positron annihilation techniques of the type described above have been successfully used in the laboratory to detect defects in specimen materials, the technique has not been successfully utilized in field settings. For one thing, the positrons from the external positron source barely penetrate the surface of the material being tested. Consequently, such external positron source techniques are limited to near surface measurements and generally must be conducted under controlled laboratory conditions.
Partly in an effort to solve the depth limitations of the foregoing positron annihilation testing technique, another type of positron annihilation technique has been developed that replaces the external positron source with an external neutron source. Neutrons from the neutron source are directed toward the material being tested. Given sufficient energies, the neutrons will, in certain materials, result in the formation of isotopes that produce positrons. Such isotopes are commonly referred to as positron emitters. The positrons then migrate to lattice defect sites, ultimately annihilating with electrons to produce gamma rays. The resulting gamma rays are thereafter detected in the manner already described in order to derive information relating to the structure of the material being tested.
The foregoing type of positron annihilation system is often referred to as a “neutron activated positron annihilation system” since it utilizes neutrons to trigger or induce the production of positrons. Since neutrons penetrate more deeply into the material being tested than do positrons alone (e.g., from an external positron source), such neutron activated positron annihilation systems are generally capable of detecting flaws deep within the material rather than merely on the surface. Unfortunately, however, only a relatively few elements, such as certain isotopes of copper, cobalt, and zinc, produce positrons in response to the neutron bombardment that are suitable for detecting flaws within the material. Consequently, neutron activated systems are limited to use with materials that contain such responsive elements.